Near-Infrared Fourier Transform Raman Spectroscopy of Photolabile Organocobalt B₁₂ and Model Compounds. 3. Vibrational Assessment of Factors Affecting the Co-C Bond in Models

Near-infrared Fourier transform (FT)-Raman spectra have been measured for a large number of B₁₂ model compounds containing the (DH)₂ equatorial ligand system (DH = monoanion of dimethylglyoxime) in order to assess the importance of various factors (viz., trans electronic effect, trans steric effect, environmental effect) that influence the Co-C bond stretch. The CO-CH₃ stretching mode of these B₁₂ models in the solid state is generally detected as a very intense and sharp Raman line at ca. 500 cm^-1, which exhibits a frequency decrease of 2-27 cm^-1 in chloroform solution. Comparison of FT-Raman results with X-ray structural data indicates the existence of structural differences between the solid state and solution. It is suggested that the flexible Co(DH)₂ unit is bent from planarity more in solution than in the solid. Such a conformational distortion should lead to a weakening of the Co-C bond. Although there is a well-established relationship between the Co-C bond cleavage rate of (4-X-pyridine)Co(DH)₂R complexes and 4-X-pyridine basicity, FT-Raman studies of a series of (4-X-pyridine)Co(DH)2CH3 compounds (X = H, cyano, tert-butyl, and NMe₂) in chloroform unambiguously reveal the absence of a trans electronic influence on the Co-C stretching frequency in the ground state. A similar study of a series of PR₃Co(DH)₂CH₃ complexes (R = methyl, n-butyl, phenyl, and cyclohexyl) confirms the presence of a trans steric influence. The observed trans steric influence in the ground state, however, is not large enough to account for the differences in Co-C bond cleavage rates for related B₁₂ models with bulky axial alkyl groups. These findings suggest that the energetics of the transition state/ground state properties not directly related to Co-C bond strength are important. Since the v_(Co-c) stretch in these models is similar to that in the coenzyme, methyl B₁₂, we discuss the possibility that the B₁₂-dependent enzymes enhance the Co-C bond cleavage rate by lowering the overall energy of the transition state, rather than by significantly weakening the Co-C bond in the ground state.